Electrical machines such as motors, generators, alternators or the like typically consist of a rotor and a stator surrounding the rotor and coaxial therewith. The rotor and stator each generally comprise a core, around which electrical wires are wound to form the magnetic coils of the device. Generally, the rotor core comprises a cylindrical metallic component having external longitudinal grooves into which the winding wires may be laid. The annular stator core has longitudinal grooves on its inside surface, into which the stator windings are laid.
Laminated cores for electrical machines have been produced by stamping core parts from sheet metal, and then assembling the stamped parts in a stack to form electric motor cores.
In this process, a sheet or strip of metal is stamped to form annular parts for stator cores, and disc-like parts for rotor cores. The stamping process produces large amounts of scrap, and a first attempt to mitigate the production of scrap was made by first stamping the disc-like parts for the rotor core from the sheet material, and then stamping the annular shaped parts which form the stator core from the area of the sheet material surrounding the opening formed by the removal of the rotor core part. A consequence of this technique to reduce scrap, however, is that the rotor and the stator cores are formed from the same metal sheet or strip, and thus must have the same magnetic properties.
To permit the rotor core and the stator core to have different magnetic material properties, techniques have been developed to produce rotor and stator cores separately. The disc-like rotor cores are stamped from metal sheet or strip as before. In order to reduce the amount of scrap generated when producing the annular cores for the stators, production methods for annular cores have been developed in which a strip of metal formed with a castellated edge is wound in a helix to form an annular laminated core.
Examples of this method can be found in, for example, U.S. Pat. No. 4,395,815 which describes forming a laminated stator core by stamping openings in one edge of a metal strip, forming the metal strip into a helical form with the openings on the radially inner edge of each coil, and securing the helical windings of the coiled strip together using longitudinally-extending bolts passing through the laminations of the core adjacent its outer edge.
In order to form the stamped strip into an annular stator core, the current conventional technique is to wind the strip onto a generally cylindrical winding mandrel to form the laminated core, and then grip the wound core and transfer it to a sizing mandrel of a slightly larger diameter than the winding mandrel. The use of a sizing mandrel is necessary because, during winding of the strip, irregularities in the thickness or material properties of the strip can result in the slots or openings formed in the edge of the strip becoming slightly misaligned in the wound coil. The transfer is effected by gripping the wound core and pulling it axially from the winding mandrel, and then placing the core axially onto the sizing mandrel. The sizing mandrel typically has a tapered leading end to assist in inserting the sizing mandrel within the coil.
The sizing mandrel conventionally has longitudinally-extending ridges on its outer surface to engage with the openings on the inner surface of the wound core, to ensure accurate alignment of adjacent coils of the core. The sizing mandrel, due to its slightly larger diameter, applies a tensile strain to the windings of the coil as the coil is placed on the mandrel, slightly stretching the strip material of each coil while the ridges cooperate with the openings in each coil of the strip to ensure accurate alignment.
It will be appreciated that significant amounts of energy are stored within the wound coils of the core, and even for small-diameter cores the equipment required to grip and hold the wound strip during the transfer to the sizing mandrel is considerable. Likewise, significant force is required to place the wound coil onto the sizing mandrel, due to the stretching of the coils during this operation.
The coils of the winding are then fixed together to form the laminated core, preferably by welding at the outer edges of the core. The finished laminated core is then removed from the sizing mandrel, an operation which again requires significant force due to the tight fit between the now-finished core and the sizing mandrel.
A person of ordinary skill in the art will readily appreciate that the conventional process described above can be used only to produce stator cores which have axially-extending internal grooves to receive the motor windings.